Chemical milling



July 3, 1962 J. A. HARDY 3,042,556

` CHEMICAL MILLING "WSN/WAN E1n/a @N1-mw INVENTOR.

Johy) HOV/d3 BY Thomas M/SeCfeS-t July 3, 1962 5 Sheets-Sheet 2 Filed Sept. 22, 1958 hO d om om ,QN QN E i L2 Ci www BLV u 9 Nl IQHO J. A. HARDY CHEMICAL MILLING July 3, 1962 5 Sheets-Sheet 3 Filed Sept. 22, 1958 mo mOda Q.- md

, INVENTOR. John A Hard BY July, 1962 J, A, HARDY 3,042,566

CHEMICAL MILLING Tlfwwm s W. Sur-est J. A. HARDY CHEMICAL MILLING July 3, 1962 5 Sheets-Sheet 5 Filed sept. 22, 1958 F I G. IO

PHosPHoRL/s PENTox/DE (PER CENT By wE/c-:HBY .Joh n A. H a d3 Th o mas MSecresZ' United. States Patented July 3, ld!! 3,042,565 Clfmll/HCAL MlLLlNG John A. Hardy, Seattle, Wash., assigner to Boeing Airplane Company, Seattle, Wash., a corporation of Delaware .5

Filed Sept. 22., 1953, Ser. No. 762561 S Claims. (Cl. d-i8) This discovery relates to the chemical milling of metal, ceramics and cermets and, more particularly, to the chemical milling of the ceramic alumina in an acid medium.

Prior to specifically discussing cher .ical milling l wish to point out the manner in which it distinguishes from pickling, brightening, decorative design and surface increase. Chemical milling may be considered to be controlled corrosion or controlled metal removal to form Sculptured metal congurations. In chemical milling a relatively large percentage of Athe metal may be rapidly removed so as to leave a minor amount of the original metal in a new configuration. As contrasted with this is pickling or scale removal whereby as much as possible of the oxide and other coating of the metal are removed but as small amount of the metal as possible is removed. ln other words in pickling only the surface coating of the metal is removed. ln brightening or surface polishing a minimum amount of the metal is removed to form a reflective surface as the scale has been previously removed; Decorative designs are of a shallow depth with approximately ve mils of surface removed `-by chemical action. in decorative design part of the surface is masked and the design is inscribed through the masked coating. Next, a chemical solution is applied to the masked material and allowed to react with the metal to form the design. An 3 example of decorative design is silverware. In surface increase the object is to form a roughened surface. ln order to specifically increase the surface area the same is purposely pitted by chemical action. This is done to make the surface rough enough for bonding purposes such as a metal to metal bond, a base for paint and the manufacture of condenser plates to name a few of the uses.

The mechanical milling of metal is well-known and extensively used throughout the metal working industry. Even though mechanical milling is extensively used it inherently has certain limitations. One of these limitations is the expensive capital investment for milling machines. Such la machine, capable of making large, complicated configurations, may cost in the hundreds of thousands of dollars. in addition to the capital investment it is necessary to have skilled Workmen operating these machines in order to do a creditable job on the milling of metal. Also, because of cost and man-hour requirements this type of milling is limited to certain configurations, namely, those configurations which are of a more simple or open design. However, by `the skill-ful use of welding it is possible to build these limited configurations into an intricate design. Because of the nature of the weld and the inherent weakness therein it is not always advisable to build these limited basic coniigurations into an intricate design as the weld might rupture or the added weight become excessive. Furthermore, with mechanical milling there usually results a relatively :rough surface. Because of excessive cost the tolerance, in a recessed area, is not a close one but must .be .a rather large one such as in the hundredths of an inch. Also, on some of the newer metals it is impractical, because of cost due to equipment and to the time of milling involved, to mechanically mill the surfaces as the metal is extremely tough and resistant to mechanical cutting and/ or rapid abrasion. For example, only casting and rough grinding can 4be employed with certain metals.

The prior art is not concerned with the chemical milling of cerrnets and ceramics such as alumina, aluminum oxide, but is more concerned with the polishing of such a ceramic. For example, because alumina is such a hard material one of the techniques of polishing employs diamond dust or diamond powder. As is well-known diamond is the hardest material and therefore can be used for polishing a softer material such as alumina. However, the polishing of alumina with diamond powder leaves the article covered with minute scratches which act or function as stress raisers so as to decrease the ultimate strength of alumina. The patents to Espig et al., Numbers 1,806,588 and 1,806,589, issuing date of May 2.6, 1931, teach of the polishing of alum-ina, in the form of sapphire, ruby and synthetic corundum, by borax and sodium carbonate at a temperature of about 750800 C. Graham, Patent Number 2,510,219, issuing date of June 6, 1950, teaches of the polishing of alumina with a mixture of sodium carbonate and borax at a high temperature. More 4particularly, Graham teaches of the glossing of the surface of a corundurn article by mmersing it in a mixture consisting essentially of between one and six parts by weight of sodium carbonate to one part by weight of an alkali borate such as sodium or potassium tetraborate at a temperature in the range of l480-1740 F. By such treatment it is possible to produce a surface on alumina which is substantially smooth and is free of a wavy or undulating surface.

To repeat, the prior art did not deal with chemical milling; it dealt with the polishing of alumina. As disclosed in the patent to Graham, his process is vfor the polishing of alumina vat a high temperature. This temperature is diicult to maintain and in itself is expensive to maintain because of the high heat loss due to the driving potential. With this in mind I have invented a process for milling alumina at a relatively rapid rate of approximately vetenths (0.5 rnil) mil per minute at a relatively low temperature of approximately 550-750 F. With such a rapid milling rate itis possible to economically eat away the alumina. Also, at the low milling temperatures it is possible to economically heat or maintain the liquid milling solution because of the lower driving potential.

To further illustrate some of the advantages of chemical milling it is possible to achieve the same output by chemical milling with a lower capital investment than for an equal output with mechanical milling, Also, in chemical milling the application of masking material to the part to be formed requires semi-skilled labor as contrasted with the requirement of skilled labor for mechanical milling. And, more intricate configurations can be formed more rapidly by chemical milling as the metal to be milled can be immersed in the reactant solution so that all parts of the metal can be acted upon simultaneously. In a specific illustration as applied to the manufacture of airplanes it is possible to reduce the weight of certain members of the airplane, as an example, stainless steel sheet stock may come in a thickness of 0.093 inch; however, from a design standpoint a thickness of 0.086 inch may be adequate. The original stock would not be reduced from the thickness of 0.093 inch to the thickness of 0.086 inch by mechanical milling as the cost would be too great, especially for heat treated stock. However, it is possible by chemical milling to immerse the stock in a chemical reactant solution and reduce it to the thickness desired with ease and economy. In large airplanes this reduction in thickness may mean a considerable saving in the weight of the air frame. More particularly, for every excess pound carried by the airplane there may be required from tive to ten pounds of additional gross weight air frame weight. Therefore, if in the large airplanes a thousand pounds of unneeded material can be saved the air frame weight can be reduced by approximately 5,000 pounds to 10,000 pounds.

In another manner the configuration of a chemically milled metal can be more economically achieved than by mechanical means. For example, in the case of a wale pattern, the necessary chemical milling can be relatively easily performed as the masking materials can be placed over the metal and the metal not protected by this masking material can be chemically eaten away or chemically corroded. ln a like manner an object can be tapered by removing the same slowly from the chemical milling solution. Also, an object having a large number of irregularly placed recessed areas can be readily manufactured by chemical milling while the preparation of a similar object by mechanical means would be time-consuming and dicult to prepare.

With this in mind it is an object of this discovery to provide a method for the chemical milling of ceramic, alloys and cermets, examples of the milling being intricate configurations, taper and waffle pattern.

A further object is the provision of a method for the chemical milling at a relatively .uniform and controllable rate.

A still further object is to provide a solution for the chemical milling of alumina which solution can be controlled by standard chemical analysis.

Another important object is the provision of a process for the chemical milling of metal and which process provides satisfactory surfaces having well-defined edges at better than standard machine tolerances.

A still further object is the provision of a process for chemical milling resulting in a satisfactory structural and mechanical properties in the milled metal.

A still further and important object is to provide for the chemical milling and which chemical milling results in a satisfactory, smooth surface.

An additional object is the provision for a process for chemical milling and which process produces a minimum of smut.

Another object is the provision of a process for chemical milling which is less expensive than mechanical milling and makes it possible to produce more intricate congurations than mechanical milling.

Another object is the provision of a process for chemical milling which can be carried out at a lower temperature than previous processes employed for the polishing of alumina.

Another object is the provision of a process for chemical milling which is free of intergranular corrosion.

Another important object is the provision of chemical milling which results in a product free of stress centers.

These and other important objects and advantages of,

the invention will be more completely brought out and CiJ Cil

understood by reference to the following detailed description of the invention and the accompanying claims.

ln the drawings: Y

FIGURE 1 is a plot of the milling rate versus the percentage of phosphorus pentoxide in the milling solution.

FIG. 2 is a plot of the milling rate versus the percentage of phosphorus pentoxide in the milling solution.

FIG. 3 is a plot of the total material milled versus total elapsed time for a number of milling solutions containing phosphorus pentoxide.

FG. 4 is a plot of the milling rate versus the weight ratio of sulfur trioxide to phophorus pentoxide for two milling solutions.

FIG. 5 is a plot of the milling rate versus the weight ratio of sulfur trioxide to phosphorus pentoxide for a milling solution.

FIG. 6 is a plot of the milling rate Versus the weight ratio of phosphorus pentoxide to sulfur trioxide for a milling solution.

FIG. 7 is a plot of total material milled versus total elapsed time for two milling solutions containing phosphorus pentoxide. y

FIG. 8 is a plot of total material milled versus total elapsed time for a number of milling solutions containing phosphorus pentoxide.

FlG. 9 is a plot of the milling rate versus the weight ratio of sulfur trioxide to phosphorus pentoxide for a milling solution; and

FiG. l0 is a composite plot of the milling rate versus the percentage of phosphorus pentoxide for milling solutions containing phosphcrus pentoxide and both sulfur trioxide and phosphorus pentoxide.

Briefly, this invention relates to the chemical milling of a ceramic such as alumina in an acid medium comprising phosphoric acid and phosphorus pentoxide, phosphoric acid and phosphorus pentoxide and mercury, phosphoric acid and sulfuric acid, phosphoric acid and phosphorus pentoxide and sulfuric acid, sulfur trioxide and mercury. The chemical milling is carried out at relatively low temperature in the range of about S50-750 F.

As previously stated the acids employed in the invention are phosphoric and sulfuric acids. The phosphoric acid may be concentrated phosphoric acid or may be the product of phosphoric acid and phosphorus pentoxide. Briefly, concentrated phosphoric acid Vis orthophosphoric acid, H3PO4. Orthophosphoric acid with the addition of phosphorus pentoxide may be transformed into pyrophosphoric acid, H4P2O7. With the further addition of phosphorus pentoxide there may be formed metaphosphoric acid, HPO3. Briefly, concentrated orthophosphoric acid, about 86% H3PO4 and the rest Water, and phosphorus pentoxide may give pyrophosphoric acid, about 79.8% phosphorus pentoxide by weight, and concentrated orthophosphoric acid with a larger concentration of added phosphorus pentoxide may give metaphosphoric acid, about 88.7% phosphorus pentoxide by weight. Concentrated orthophosphoric acid by itself is approximately 62.4% phosphorus pentoxide or approximately 86% phosphoric acid with the balance substantially water. Also, there is employed in this invention sulfuric-acid, H2804. As is well-known, with the addition of sulfur trioxide to lsulfuric acid there may be formed pyrosulfuric acid, H2S2O7. Concentrated sulfuric acid is approximately 98% by weight sulfuric acid with the balance water. Or, approximately 80% by weight sulfur trioxide. Fuming sulfuric acid comprising approximately 20-22% by weight sulfur trioxide with the balance sulfuric acid is on a weight basis approximately 85.3% sulfur trioxide. Fuming sulfuric acid comprises sulfuric acid, free sulfur trioxide and some pyrosulfuric acid. As it is not possible to definitely state the chemical composition of the chemical milling solutions employed because it is not known what are the percentages of orthophosphoric acid, pyrophosphoric acid and metaphosphoric acid and other phosphorus acids, sulfur acids and sulfur trioxide in the acid mixtures and it is not known the compositions resulting, I will express these compositions in terms of phosphorus pentoxide and sulfur trioxide. In that manner the amount of active material is stated, I believe, with a higher degree of precision than by stating concentrated phosphoric acid and filming sulfuric acid or the like.

The alumina materials milled by these milling solutions were alumina prepared by The Boeing Airplane Company, Seattle, Washington, and comprising on a weight basis about 96% Al205 (alumina), 2% MnO (manganese oxide) and 2% Ti02 (titanium dioxide); and Burundum, a product of The United States Stoneware Co., Akron, Ohio, and comprising about 88.25% A1205 (alumina) and the balance potassium oxide, sodium oxide, barium oxide, silica, calcium oxide, magnesium oxide, iron oxide and traces of other metal oxides.

A milling solution was prepared by adding phosphorus pentoxide to a concentrated phosphoric acid to prepare a solution comprising approximately 79.8% by Weight of phosphorus pentoxide. n the theoretical basis this weight of concentrated phosphoric acid plus the phosphorus pentoxide should become 100% pyrophosphoric acid. This solution was employed to mill alumina or aluminum oxide (about 96% A1203, 2% M110 and 2% Ti02). The alumina was placed in the milling solution at three different temperatures, i.e, 550 F., 600 F., and about 720 F. At the two lower temperatures the milling rate was very low, being in the order of tive-hundredths (0.05) mil per minute. At the higher temperature of approximately 720 F. the milling rate was increased substantially to the order of approximately forty-tWo-hundredths (0.42) mil per minute. At this temperature the solution is in the state of a ybrisk boil. As expected, an increase in temperature results in a higher milling rate. From this data it is possible to draw the conclusion that a minimum temperature is required in order to have an appreciable milling rate. This minimum temperature must be above approximately 600 F. in order to realize the milling rate of any value. The data from these runs is in Table I, which follows:

TABLE I Elapsed Tempera- Milling time, ture, Rato, minutes F. mils/min.

A series of mixtures was prepared by adding phosphorus pentoxide to concentrated phosphoric acid. For example, there were prepared mixtures comprising phosphorus pentoxide as indicated `by the following approximate percentages: 62.4% (concentrated orthophosphoric acid), 72.4%, 75.8%, 79.8% (cor-responds to pyrophosphoric acid mixtures), 81.8% and 88.7% (corresponds to metaphosphoric acid mixtures). In addition, a solution comprising about 79.8% pyrophosphoric acid there were added a few drops of mercury to see the elect of this element on the milling rate. These mixtures were employed to mill alumina (about 96% A1203, 2% M110, and 2% no2). The alumina was placed in the mixture forty-seven hundredths (0.47) mil per minute.

TABLE II Milling Solutions Comprising Concentrated Phosphoric Acid Plus Phosphorus Pentoxide Weight percent Tempera- Milling P205 in mixture ture, F. rate mils (approx.) per min.

Norm-Total elapsed time-30 minutes.

This data is graphically presented in accompanying FIG- URE l. As is seen from this graph, concentrated phosphoric acid, approximately 62.4% P205, does not appreciably react with the alumina. Also, metaphosphoric acid, approximately 88.7% P205, does not react appreciably with the alumina. However, between these two concentrations of P205 the mixtures react with the alumina and for a mixture comprising approximately 79.8% P205, i.e., corresponds theoretically to pyrophosphoric acid, there is reached a milling rate of approximately 'Also, at the concentration of approximately 79.8% P205 in the mixture and With the addition of mercury thereto it is noticed that the milling rate increased appreciably, approximately one-third, to ybe about sixthy-fou-r hundredths (0.64) mil per minute. However, even with this increase in the milling rate on the addition of mercury it was observed that the milling temperature decreased from approximately 720 F. Therefore, it is possible to state that with the addition of mercury or a member of the sub-group of the second series of the periodic table, i.e., zinc, cadmium and mercury, that there is a decrease in the milling temperature and a corresponding increase in the milling rate.

Another series of reaction mixtures was prepared comprising concentrated phosphoric acid plus phosphorus pentoxide. In addition another mixture comprising concentrated phosphoric acid, phosphorus pentoxide and mercury was prepared. These mixtures comprised on a percentage weight basis the following indicated approximate percentages of phosphorus pentoxide: 75.8%, 77.8%, 79.8% and 81.8%. Burundum was placed in these mixtures for various periods of time and the degree of chemical milling was determined for periods of one hour in the mixture held at a brisk boil. The temperature of reaction for these mixtures was in the range of approximately 60G-750 F. The reaction temperature for the mixture comprising concentrated phosphoric acid, phosphorus pentoxide and mercury was about 550 F. or approximately F. lower than the other reaction temperatu-res. This data is presented in the following Table III and is illustrated in FIGURE 2.

accese@ 7 TABLE m Milling Solutions Comprising Concentrated Phosphoric Acid Pitts Phosphorus Pentoxidc Weight percent Tempera- Milling P205 in mixture ture, F. rate, mils (approx.) per min.

75. 8 600 0.27 77.8 660 0.40 79. 8 720 0.49 81.8 750 0. 33 79. S-i-Hg 550 0. 64

Nomar-Total elapsed time- 60 minutes;

tem a desirable one from the economic standpoint of Vchemical milling as well as from a safety feature.

Similar solutions to those presented in Table III were prepared and Burundum milled in them. These solutions were reiluxed so as to maintain, except for the reaction of the solutionkwith the Burundurn, the original solution. More particularly, these solutions comprised concentrated phosphoric acid plus phosphorus pentoxide so as the following the approximate Weight percentages of phosphorus entoxide; 75.8%, 77.8%, 79.8%, 81.8% phosphorus pcntoxide and 79.8% phosphorus pentoxide plus a small amount of mercury. These solutions were allowed to react with the Burundum for various periods of time Varying from one hour to four hours. The milling temperature range for these solutions was approximately 60G-750 I?. except the one comprising mercury which was approximately 550 F. A summary of this data is presented in Table IV which follows:

TABLE IV Milling Solutions Comprising Concentrated Phosphoric Acid Plus Phosphorus Pentoxide Weight Re- Total Iota-l Temp. percent action reaction Milling, milling, F. P205 in time, time, mils nuls (apmixture min. min. prox.)

30 30 8. 5 8. 5 G00 75.8 30 G0 7. 5 16. 0 600 180 240 25. 0 4l. 0 600 30 30 1l. 5 l1. 5 660 30 60 l2. 5 24. 0 660 77. 8 30 90 12. 0 36. 0 660 30 120 8. 5 44. 5 660 30 150 5. 5 50. 0 660 30 30 20. 5 20. 5 550 79. s|Hg ao 6o 18. 0 3s. 5 550 The data in Table IV is plotted in FIGURE 3. An examination of this gure graphically illustrates the relative reaction for the period of one hour of elapsed time. The reaction mixture comprising about 79.8% phosphorus pentoxide plus mercury had the most rapid reaction rate and milled the largest quantity of material for a given time period. The next most reactive milling solution comprised approximately 79.8% phosphorus pentoxide.

8 The other milling mixtures were not as reactive as these two. Also, it is to be noted from FIGURE 3 that the milling rates were substantially constant forythe mixture comprising phosphoric acid and 79.8% phosphorus pentoxide, both with and without mercury. This is the same for the mixture comprising about 77.8% phosphorus pentoxide and for a total elapsed time of one and one-half hours. Again, from this data it is possible to state that the milling solution comprising about 79.8% phophorus pentoxide, Le., theoretically equivalent to pyrophosphoric acid, possesses the fastest milling rate on Burundum and that the addition of mercury increased the milling rate while decreasing the milling temperature. Also, from this data it is possible to draw the conclusion that the milling solutions possess acons'tant milling rate.

This latter feature is of especial value as in the milling of an object it is possible to accurately estimate the degree of milling and therefore come out with a finished object having a stricter tolerance. lf it were not possible to accurately estimate the milling rate then it would not be possible to control the tolerance within as high a degree as with the accurate estimation.

Another milling solution was prepared comprising fuming sulfuric acid, concentrated phosphoric acid and phosphorus pentoxide. This solution was employed to mill Burundum at three different temperatures. The chemical milling was carried out under reflux conditions. This solution comprised one part of turning sulfuric acid and` TABLE V Milling Solution Comprising Sulfuric Acid, Phosphoric Acid and Phosphorus Pentoxide [Ratio of 1 3 fuming sulfuric acid: concentrated phosphoric ac1d plus phosphorus pentoxide] Percent Percent by weight by weight of Milling rate, Temp., of sulfur phosphorus mils/min. F

trioxide pentoxide (approx.) (approx.) (approx.)

Asis seen at a temperature of approximately 550 F. the milling rate is substantially zero. At a temperature of approximately 600 F. the milling rate is about 0.185 mil per minute. And, at a temperature of about 700 F. the milling rate is approximately 0.57 mil per minutes. From this data it is possible to draw the conclusion that at a lower temperature the reaction mixture is not capable of milling alumina while at a higher temperature of approximately 700 F. the mixture is capable of relatively rapidly milling alumina. Also, it is possible to state that there must be a minimum temperature of the reaction mixture in order to mill aluminum oxide and below which temperature milling will not take place.

A series of milling solutions was prepared comprising concentrated sulfuric acid and concentrated phosphoric acid in two ratios and futnng sulfuric acid and concentrated phosphoric acid in two ratios. These milling solutions Were used to mill Burundum under conditions where vapors from the Solutions could escape. As a result the compositions of the solutions varied andthe boiling points varied from about 430 F. to approximately 720 F. This data is presented in following Table VI.

aoaaece Milling Solutions Comprising S Ltlfurc Acid and Phosphoric Acid Percent Percent Weight Elapsed Milling Temp., Ratio f H2504: HzSO.; H3? O4 S03 P205 l'atiO time, rate, F.

HsPOi by weight by weight S Oa/PzOs min. mils/min. (approx.)

(approx.) (approx.) (approx.)

This data is graphically illustrated in FIGURE 4. In FlGURE 4 the milling rate in mils per minute is plotted versus the weight ratio of sulfur trioxide to phosphorus pentoxide. Referring to the milling rate variation for the mixture comprising concentrated sulfuric acid and concentrated phosphoric acid it is seen that as the concentration of the phosphorus pentoxide increases with respect to the sulfur trioxide that the milling rate rapidly increases. Also, in the reaction mixture comprising turning sulfuric acid and concentrated phosphoric acid it is seen that the reaction rate increases. However, in FIGURE 4 it is seen that the reaction rate does not increase as rapidly in the mixture comprising fuming sulfurie acid and concentrated phosphoric acid as for the mixture comprising concentrated sulfuric acid and concentrated phosphoric acid. Even though the reaction rate for the -former mixture does not increase as rapidly -as the reaction rate for the latter mixture the reaction rate of the former is larger than the reaction rate of the latter.

Another series of milling solutions was prepared cornprising sulfuric acid and phosphoric acid. These solutions were employed to mill Burundum. ln this series three of the mixtures comprised fuming sulfuric and concentrated phosphoric acid, one mixture comprised concentrated sulfuric and concentrated phosphoric acid, and one mixture was concentrated phosphoric acid by itself. The milling temperature was in the range of approximately 325-770 F. and the milling time was approximately thirty minutes except in one instance where it was thirty-live minutes. The variation in the milling temperature was due to the mixture boiling away or vaporizing so as to have a mixture of Varying consistency and properties. These milling mixtures were 'held at a brisk boil which resulted in an increase in percent concentration of phosphorus pentoxide in the solution because of a greater loss of water from the solution than phosphorus pentoxide from the solution during boiling. Boiling concentrated orthophosphoric acid will not mill alumina until considerable Water, etc. has boiled away.

TABLE VII Referring to the above table and also to FIGURE 5, it is seen that with concentrated phosphoric acid that the milling rate was approximately 0.034 mil per minute. With the addition of fuming sulfuric acid to the concentrated phosphoric acid in the ratio of one part sulfuric acid to two parts phosphoric acid the reaction rate increased considerably to a value in the range of approximately 0.4 mil per minute. With the further addition of turning sulfuric acid so that the ratio was 1:1 and 2:1 of fuming sulfuric acid to concentrated phosphoric acid the ratio decreased to 0.23 to 0.09 mil per minute, respectively. Also, the Burundum was milled with the 1:1 mixture ot concentrated sulfuric acid to concentrated phosphoric acid and the milling rate was approximately 0.09 mil per minute. From this data it is possible to conclude that a mixture of filming sulfuric and concentrated phosphoric is more active in milling alumina than concentrated phosphoric by itself. Also, it is possible to state that Within reasonable limits the higher the ratio of the phosphorus pentoxide in the mixture to the sulfur trioxide in the mixture the faster the milling rate. With an increase in the concentration of the sulfur trioxide in the mixture the milling rate decreased. Finally, a comparison of a solution comprising fuming sulfuric acid and concentrated phosphoric acid with a mixture comprising concentrated sulfuric and concentrated pho"- phoric shows that the former milled much faster, approximately two and one-half times as fast, as the latter. Therefore, a mixture comprising sulfur trioxide and phosphorus pentoxide mills faster than a mixture or solution of phosphorus pentoride alone and with an increase in the concentration of the phosphorus pentoxide, within reasonable limits, the milling mixture mills faster than with a mixture comprising a relatively high ratio of sulfur trioxide to phosphorus pentoxide.

Another series of milling mixtures was prepared cornprising sulfuric acid and phosphoric acid. These mixtures were used to mill Burundum at temperatures in the range of approximately 40G-760 F. and for a time of approximately thirty minutes. The variation in the milling temperature was due to the mixture being in an Milling Solutions Comprising Szilfuiic Acid and Percent Percent Weight Elapsed Milling Temp., Ratio of H2804: H2804 HaPOi a 2 s ratio time, rate, F.

HsPOl by Weight by weight S O3/P2 O5 min. mils/min. (approx.)

Conc 62. 4 0 30 0. 034 B25-770 l: Conf Cout1 40 31. 2 1. 28 30 0.09 40G-735 l: Fuming.-- Conc 28. 4 41. 6 0. 69 30 0.40 425-750 1: d0 Conc 42. 7 3l. 2 1. 37 35 0.23 50G-750 2: -do Conc 56. 9 20. 8 2. 73 30 0.09 535-760 aca-.ases

141 open vessel so as to be free to boil away or vaporize. Therefore, the consistency of the mixture continually varied. These milling m'mtures were held at a brisk boil. The milling rates were determined for the entire thirty minutes. The ratios of the acids were varied to determine the effect of the various concentrations. This data is presented in the following Table VIII and is graphically illustrated in FIGURE 6.

variation in the milling temperture was due to the mixture being in an open vessel so as to be free to boil away or vaporize. Therefore, the consistency and the properties of the mixture continually varied. These milling mixtureswere held at a brisk boil. There Were various milling time periods. In one example both the Burundum and 96% aluminum oxide were milled for approximately sixty (60) minutes. The milling rate for TABLE VIII Milling Solutions Comprising Sulfnric and Phospltorc Acids Percent Percent Weight Elapsed Milling Temp., Ratio of H2804: H2804 HQPO.; S03 i s ratio time, rat F.

HzPOi by weight by weight P2 Ot/S O3 min. mils/min. (approx.)

Cone 0 0 30 700 Fuming..- 0 O 30 700 1:1- Crm(l 31.2 0.78 30 0.09 400-735 2:1 Fumim1r 20. 8 0. 37 30 0.09 535-760 1:1-- do 31.2 0. 73 30 0.23 500-750 1 :2 -do 41.6 1. 47 Y 30 0, 40 425-750 From this table and also from the corresponding ligure it is seen that both furning sulfuric acid and concentrated sulfuric acid at a brisk boiling temperature did not mill Burundum. With a 2:1 ratio of fuming sulfuric acid to concentrated phosphoric acid the reaction rate increased to approximately 0.09 mil per minute. With a change of the fuming sulfuric acid to concentrated phosphoric acid ratio to 1:1 the reaction rate was again increased to about 0.23 mil per minute, and With the change in the concentration of phosphorus pentoxide so that the ratio of fuming sulfuric acid to the concentrated phosphoric acid Was 1:2 the milling rate increased to `approximately 0.4 mil per minute. From FIGURE 6 it is seen that Within reasonable limits that with an increase in the concentration of the phosphorus pentoxide that the reaction 'rate increases substantially linearly. To observe 4g the effect of concentrated sulfuric acid and concentrated phosphoric acid there was prepared a 1:1 mixture of these acids and the milling rate on Burundum was 0.09 mil per minute. This milling rate of the fuming sulfuric acidconcentrated phosphoric acid mixture was considerably 4 faster than the milling rate of a mixture comprising concentrated sulfuric `acid and concentrated phosphoric acid in a 1:1 ratio. From this data it is possible to state that the reaction rate is approximately a linear function of the weight ratio of the phosphorus pentoxide to sulfur 50 trioxide and that With a decrease in the strength or concentration of the phosphorus pentoxide that the milling rate decreases.

Another milling solution was prepared comprising on a weight ratio one part of fuming sulfuric acid and two parts of concentrated phosphoric acid. This mixture was employed to mill Burundum and aluminum` oxide comprising approximately 96% alumina, 2% manganese oxide and 2% titanium oxide. The milling temperature was approximately in the range of 42S-7SO F. The

this period of time was approximately 0.29 mil per minute. This is presented in Table IX. Ninety-six (96%) percent aluminum oxide was milled for thirty minutes, sixty minutes and ninety minutes. The milling rate for each of these intervals of time, i.e., the first thirty minutes, the second thirty minutes and the third thirty'minutes, is presented in Table IX. For the first thirty minutes the milling rate was approximately 0.30 mil per minute, then 0.29 mil per minute and 0.12 mil per minute. From this data it is seen that there was a substantially constant milling rate for approximately sixty (60) minutes but after that period of time the milling rate decreased appreciably. Burundum was also milled for approximately a thirty minute interval, another thirty minute interval and a third thirty minute interval. The milling rate for the first thirty minute interval Was approximately 0.25 mil per minute, the second thirty minute interval was about 0.24 mil per minute and the third thirty minute interval was about 0.10 mil per minute. Again, it is seen that for approximately the rst hour of milling that the milling rate on Burundum was substantially constant, but that after one hour of milling the milling rate decreased considerably. A possible explanation for this is that the active components in the milling solution Were near exhaustion, i.e., the volume of the milling solution to the material being milled was too small. Therefore, by increasing the volume or by adding fresh milling solution a more constant milling rate may be realized. Also, from this data it is seen that the milling rate on 96% aluminum oxide was slightly greater, about 20% greater, than the milling rate on Burundum for the first hour. However, for the third half hour interval the milling rate was substantially the same and decreased to about the same value of about 0.10 mil per minute. This data is presented in following Table IX.

TABLE IX Milling Solutions Comprising Farming Sulfuric Acid and Concentrated Phosphoric Acid in the Ratio of 1:2

Percent Percent Weight Elapsed Milling Temp., F. Material milled S03 by P205 by ratio time, rate, mils/ (approx.)

Weight Weight SC3/P205 min. min.

Burundum and 96% A1203... 28. 4 41. G 0.81 60 0. 29 425-750 96% AlzOa 28. 4 4l. 6 0. 81 30 0. 30 425-750 60 0. 29 425-750 90 0. 12 425-750 Bnrundum 28. 4 41.6 0. 8l 30 0. 25 425-750 60 0.24 425-750 90 0.10 425-750 Another series of milling solutions comprising one part of fuming sulfuric acid to three parts of concentrated phosphoric acid and varying parts by Weight of phosphorus pentoxide based on the total weight of the sulfuric and phosphoric acids were prepared. These milling mixtures were employed to mill Burundum at temperatures of approximately 50G-740 F. in a reflux system. At these temperatures the mixtures were at a brisk boil. The Burundum was milled for a period of approximately thirty (30) minutes, taken from the reaction mixture, and the differential milling measured. Then the Burundum object Was again placed in the milling mixture and allowed to mill for approximately thirty (30) minutes, taken from the mixture and the degree of milling determined. The milling period was normally thirty (30) minutes although milling times of sixty, forty, thirty-eight and one hundred and ve minutes were also employed. The milling mixtures comprised approximately 54.3%, 56.85%, 58.35%, 59.85% and 60.35% by Weight of phosphorus pentoxide based on the combined weight of the TABLE X 14 which can be interpreted to mean that it is possible to accurately estimate the degree of milling of a part. For periods of time longer than about two hours the mixture comprising mercury decreased in the milling rate, that is, the total milling increment became less and less with the passage of time. In reviewing FIGURE 7 it is to be remembered that the mixture comprising mercury milled at a faster rate and also at a lower temperature than the mixture not comprising mercury. Referring to FIG- URE 8 there are plotted the mixtures in terms of total milling versus total elapsed time, i.e., mils versus hours, of Table X. However, the mixture comprising mercury is not plotted. It is seen that the mixture comprising approximately 58.35% phosphorus pentoxide had the fastest milling rate. The mixtures comprising approxi- -mately 59.85% phosphorus pentoxide and about 60.35% phosphorus pentoxide had essentially the same milling rates. The mixture comprising about 56.85% phosphorus pentoxide had a lower milling rate. However, `an examination of FIGURE 8 shows that the total milling for these four mixtures was nearly the same. The mixture comprising 54.3% phosphorus pentoxide had a lower milling rate than the other mixtures. Also, the milling rate of this latter mixture decreased with the passage of time. Therefore, it is possible to state that the first four mixtures or those having a larger concen- Milling Solutions Comprising One Volume Fumng Sulfurc Acid to Three Volumes Concentrated Phosphorz'c Acid Plus Phosphorus Pentoxifle Time, minutes Milling, mils/minute Percent by Percent by Weight Temp., F. weight P205 Weight Ratio (approx.)

S03 SC3/P205 Diner- Elapsed Dlterental Elapsed ential 13. 6 40 40 15. 0 15. 0 700-720 5S. 85 0. 206 30 70 9. 25 24. 25 700-720 30 100 11. 0 35. 25 TO0-72() 30 130 9. 75 45. 0 70D-720 30 30 18. 5 13. 5 (i90-715 58. 13. 5 D. 204 30 50 13. 0 26. 5 (590-715 30 90 16. 0 42. 5 (i90-715 30 120 l0. 0 52. 5 GSO-715 30 30 l0. 0 l0. 0 680-710 59. a5 1s. 4 o. 2o so e0 12. o 22. o eso-71o 120 28. 0 50. 0 (S80-710 30 150 10. 5 60. 5 680-710 80 180 l1. 5 72. 0 680-710 30 30 10. 75 10. 75 730-740 so. 35 13. 3 0.199 a0 so 12.5 23. 25 730-740 60 120 24. 5 47. 75 730-740 38 38 18. 5 18. 5 50G-550 5s. ssa-Hg 13. s 0. 206 30 es 12.0 so. 5 50o-550 30 98 13. 5 44. 0 50G-550 60 158 19. 0 63. 0 500-550 60 218 17. 5 80. 5 50G-550 Referring to FGURE 7 there is plotted the total milling in mills versus the total elapsed time in hours for the milling mixture comprising one part of uming sulfuric acid, three parts concentrated phosphoric acid and 56.85% phosphorus pentoxide based on the combined weight of the sulfuric and phosphoric acids. Also, one of these curves represents the milling by a mixture having a small amount of mercury. A comparison of these two mixtures shows that the mixture comprising mercury milled at a faster rate than the mixture not having the benet of mercury. The mixture comprising mercury was used for milling for a longer period of time than the mixture not having the mercury therein. It is seen that the milling rate of these two mixtures for a period of approximately two hours was substantially constant, 75 with respect to each other.

tration of phosphorus pentoxide had a more constant milling rate and also a faster milling lrate than the mixture comprising about 54.3% phosphorus pentoxide. From this data it is possible to state that the addition of phosphorus pentoxide increases the milling rate of a mixture comprising fuming sulfuric acid and concentrated phosphoric acid and also imparts a more constant milling rate. However, above a certain concentration the increase in the milling rate is not appreciable with the addition of phosphorus pentoxide.

Another series of milling solutions was prepared comprising sulfuric acid, phosphoric acid and phosphorus pentoxide. In these solutions the ratios of yfuming sulfuric acid lto concentrated phosphoric acid were varied These mixtures were employed under reflux conditions at a brisk boil to mill Burundum. The milling time period was approximately thirty minutes and the milling rate was measured for this `time period. The temperature of milling varied from approximately Z50-600 F. This data is presented in following Table XI.

TABLE Xl Milling Solutions Comprising Filming Sulfurc Acid and Concentrated Phosphoric Acid Weight; ratio Percent Percent Weight Time Milling Temp. H2804 to HaPOl by Weight by weight ratio elapsed, rate,

P205 S03 SOg/PQOE min. mils/min. (approx.)

4:1 (f: 12. 5 60. 6 4. 85 600 3:1 (f: 15. 7 55. 3 3. 52 585 5:2 (t: 17.9 51.7 2. 88 575 2:1 (i: 20.9 47.0 2.25 560 3:2 (f: 25. 0 41.0 1. 64 535 1:1 (f: 31. 4 32. 5 1. 04 500 2:3 (f: 37. 7 24. 8 0. 66 470 1:2 (f: 41. 9 20.0 0. 48 435 1:3 (f: 47. 3 19. 1 0. 40 395 Dil, HgPOl 50.1 0 250 Conc. 1131904..-" 62.4 0 310 NorE.-= Fuming sulfuric acid. c= Concentrated phosphoric acid.

A graphic illustration of this data is presented in FIG- URE 9. In FIGURE 9 the milling rate in mils per minute is plotted versus the weight ratio of sulfur trioxide --to phosphorus pentoxide. It is seen that the milling rate for mixtures comprising two to three parts, one to two parts, and one to three parts by weight of fuming sulfuric acid to concentrated phosphoric acid wherein the percent by Weight of added phosphorus pentoxide bring the concentration of the phosphorus pentoxide to about 37.7%, 41.9% `and 47.3% does not have an appreciable milling rate. However, for a weight ratio approximating forty-tive hundredths (0.45) sulfur trioxide to phosphorus pentoxide the milling rate is zero, but from there it increases rapidly, linearly, to approximately 0.05 mil per minute for a mixture comprising approximately 47% sulfur trioxide and 20.9% phosphorus pentoxide. For a mixture comprising approximately 51.7% sulfur troxide and about 17.9% phosphorus pentoxide the milling rate is also about 0.05 mil per minute. For higher concentrations `of sulfur trioxide and lesser concentrations of phosphorus pentoxide the milling rate decreases.

A composite illustration of the data in rTables Il and XI is presented in IFIGURE l0. In this lligure there is plotted the milling rate, mils per minute, versus the percent by weight phosphorus pentoxide. As recalled, the milling solution in Table II comprised concentrated phosphoric acid and phosphorus pentoxide. The material milled was 96% alumina, 2% manganese, and 2% titanium dioxide. For Table XI the milling solution comprised fuming sulfuric acid, concentrated phosphoric acid and phosphorus pentoxide. The material milled was Burundum. The two curves in this iigure show that the milling solution comprising a high percentage by weight of phosphorus pentoxide milled 96% alumina at a khigher milling rate than the milling solution comprising turning sulfuric acid and phosphorus pentoxide milled Borundum of about 88.25% alumina. However, it is to be pointed out that the milling temperatures for the former milling solutions, e.g., about 50G-720 F., were higher than `for the latter milling solutions, eg., about G-600 F. The temperature differences may account for some of the differences in milling rates. These two curves illustrate a maximum in milling rates for different concentrations, i.e., for concentrated phosphoric acid and phosphorus pentoxide the maximum is at approximately 79.8% by weight phosphorus pentoxide, and for turning sulfuric acid and concentrated phosphoric acid the maximum is for the range of about 17 .9-20.9 percent by weight phosphorus pentoxide and about 51.7- 47.0 percent by weight sulfur trioxide.

senting the milling results there will be stated the approximate composition of these alloys. Stellite 31 on a weight percentage basis comprises carbon (US5-0.45% )g chromium (26;5-24.5%); iron (20-0.0%); manganese (LO-0.0% nickel ULS-9.5%); silicon (1 0-0.0i); tungsten (8.0-7.0%); and, cobalt (remainder or balance). Hastelloy B on a weight percentage basis comprises carbon (0.050.0%); chromium (OJO-0.0%); cobalt (2.5-

0.0% iron (7.04.0% manganese (LO-0.0% )g molybdenum (30G-26.0%); nickel (62.0%remainder); phosphorus (0.04-0.0%); sulfur (MB-0.0%); silicon (1.0- 0.0%); and, vanadium (d0-2.0%). Multimet on a weight percentage basis comprises nickel (21.0-19.0%); cobalt (ZID-18.5%); chromium (22S-20.0%); molybdenum (S5-24.5%); tungsten (3.0-2.0%); iron (remainder); carbon (0.160.8%); nitrogen (O20-0.10%); and, columbium and tantalum (1.25-0.75% Carpenter 20 on a weight percentage basis comprises approximately, carbon (0.07% maximum); manganese (0.75%); silicon (1.00%); chromium (20.00%); nickel (29.00%); molybdenum (2.00% minimum); copper (3.00% minimum); and, iron (remainder or balance).

Two milling solutions were prepared. The first was a solution comprising concentrated phosphoric acid and phosphorus pentoxide. On a weight percentage basis ythe phosphorus pentoxide was about 79.8% by weight. This corresponded to pyrophosphoric acid. The second solution comprised one part turning sulfuric acid, three parts concentrated phosphoric acid and added phosphorus pentoxide. The added phosphorus pentoxide and the phosphoric acid corresponded to pyrophosphoric acid. These solutions were used `to mill the above indicated alloys at about 600 F. for approximately one-half hour. A summary of the results is presented in following Table XII.

TABLE XLI 6Oblonf--Milling period- 30 minutes, temperature-about These results indicate that :the chemically resistant all loys are attacked at a relatively low temperature of 600 F. by a mixture comprising phospho-ric acid and phosphorus pentoxide and by a similar mixture having incorporated therein `fuming sulfuric acid. The latter mixture appears to have milled at a somewhat lfaster rate than the former.

Other alloys were milled with a mixture of fuming sulfuric acid, concentrated phosphoric acid and phosphorus pentoxide. These alloys were N-155, Hastelloy X, Inconel, 55-316 and Hastelloy C. Hastelloy X on a weight percentage basis comprises carbon (G-0.20%); chromium (20S-23.0%); cobalt (0S-2.5%); iron (17.0- 20.0%); manganese (Oil-1.0%); molybdenum (8.0- l0.0%); silicon (O0-1.0%); tungsten (0G-4.0%); and, nickel (remainder or balance). Inconel on a weight percentage basis comprises nickel (80%; chromium (14.0%); and iron (6%). N-l55 on a weight percentage basis comprises nickel (i90-21.0%); cobalt (18.5- 2l.0%); chromium (Z0-22.5%); molybdenum (2.5- 3.5%); tungsten (l0-3.0% )g carbon (QCS-0.16%); nitrogen (U10-0.20%); columbium and tantalum (0.75- 1.25%); and iron (remainder or balance). SS-3l6 on a weight percentage basis comprises carbon (O0-0.10%); chromium G60-18.0%); nickel (10C-14.0%); molybdenum (20G-3.00% manganese (O0-2.0%); silicon (O0-1.0%); phosphorus (0G-0.04%); sulfur (0.0- 0.03%); and, iron (remainder or balance). Hastelloy C comprises on a weight percentage basis carbon (0.0- 0.08%); chromium (14j-16.5%); cobalt (O O-2.5%); iron (4.0-7.0%); manganese (O0-1.0%); molybdenum (15G-17.0%); phosphorus (0C-0.04%); surfur (0.0- 0.03%); silicon (O0-1.0%); vanadium (O O-0.35%); and, nickel (remainder or balance). The milling mixtures comprised one part of fuming sulfuric acid, three parts of concentrated phosphoric acid and phosphorus pentoxide so that the phosphorus pentoxide comprised approximately 59.85% by weight (substantially equivalent to pyrophosphoric acid). A reflux system was used for milling with the temperature being about 600 F. and the time about thirty minutes. A summary of the milling results is presented in following Table XIII.

Nora-Milling period-30 minutes, temperature-about 600 F.

Reflux system: All of the super alloys and the lstainless steels in Tables XII and XIII were tested in a refluxing system.

Other alloys and metals were subjected to the action of the milling -solution disclosed in Table XIII. For example, pure nickel started to react at approximately 250 F. Iand reacted vigorously at 400 F. An alloy comprising 99.5% molybdenum and 0.5% titanium reacted at about 350 1F. and reacted vigorously at about 450 F. Commercial titanium was vigorously attacked at about 15G-160 F. by a milling solution comprising one part of fuming sulfuric acid and two parts concentrated phosphoric acid.

Although the chemical milling has been described and illustrated in regard to milling in the liquid solution it is to be realized that the milling can be carried out in the vapor phase. In carrying out the milling in the vapor phase a milling solution is prepared and the part to be milled instead of being immersed in the liquid milling solution is not placed in said solution but instead is positioned above the solution and the solution heated so as to drive off vapors. 'Ihe vapors contact the part to be milled and thereby mill the unprotected or unmasked area of the part.

In the actual chemical milling process it is necessary to protect some of the parts or some area of the par-ts being milled from the milling solution. To do this there is applied a masking material over the part to be protected. Then the part is immersed in the milling solution for a sufficient period of time to carry out the milling operation. The maskant is removed after the milling operation has been performed. For example, the chemical milling or differential milling of an object may be accomplished in a four step process outlined as follows:

(l) Complete masking of the part to be milled.

(2) Cutting the design in the maskant.

(3) Milling the part in the milling solution to remove that section of the material not protected by the maskant.

(4) Removing the maskant from the milled part.

The part to be milled may be masked by an elastomeric or plastic type of maskant. For example, this part may be masked by polytetrafuoroethylene. In masking the part a primer coat is applied to the same. The primer coat is dried and then cured for approximately twenty (20) minutes at a temperature of about 720 F. Then a liquid solution of polytetrafluoroethylene is sprayed over or onto the primer coat. Upon spraying the liquid polytetrafluoroethylene material over the primer this coating is then dried and baked at approximately 720 F, for a period of ten to twenty minutes. It is possible. to spray a number of these coatings so as to build a coating thickness varying from approximately one mil to about ten mils or more. Upon the spraying of the liquid coating it is necessary to dry the coating and then to heat it at an elevated temperature for a definite time period so as to cure the coating. Both the primer material and the liquid coating material are manufactured by E. I. du Pont de Nemours and Company, Wilmington, Delaware, a corporation of Delaware. The primer coating is: referred to in the Du Pont catalog as Number 850-20l and the liquid polytetrafluoroethylene material is referred to as Teon 30. These Teflon materials are more completely presented and discussed in United States Letters Patent Numbers 2,562,117, 2,562,118 and 2,613,193.

Another material which may be employed as a maskant is referred to as Synar compositions. These Synar binders comprise inorganic silicates, powdered minerals, oxides, carbonates, powdered metals, silica, etc. The Synar compositions may be applied over that area of the material to be protected from the milling solutions. This.` has been more completely discussed in the above paragraph in regard to Tellen. The basis for these Synar compositions is more completely described in United States Letters Patent Numbers 2,574,902 and 2,577,485. These Synar compositions are a product of Pennsalt Chemicals Corporation of Philadelphia, Pennsylvania.

In the second step the design is cut into the maskant. 'There are a number of ways of cutting this design. One way is to cover the design on the maskant with nylon tape and sand-blast the maskant from the unprotected areas. Another way of cutting the design and removing the excess maskant is to cover the part with a removable metal template. The exposed maskant is removed by sandblasting or grinding. Either of these ways removes the maskant so as to leave unprotected the area to be attacked by the lmilling solution.

In the third step the part is immersed in the milling solution. These solutions have been discussed in detail in a previous part of this specification and therefore will not be discussed in detail at this point. However, to briey discuss this procedure, two milling solutions were prepared. These comprised on a weight basis one part i9 of fuming sulfuric acid, three parts of concentrated phosphoric acid and a Very minute amount of mercury. A part made of alumina comprising approximately 96% alumina, about 2% manganese oxide and about 2% titanium dioxide was masked. A portion of the masking material was removed so as to expose an area of the alumina part for chemical milling. In one instance the thickness of the maskant was approximately eleven mils and in the second instance the maskant was `approximately eight mils in thickness.` The milling time was about thirty minutes at a temperature of approximately 550 F. Ihe milling mixture comprised in addition to the sulfuric and phosphoric acids approximately 66.8% by weight of phosphorus pentoxide so as to give a weight ratio of sulfur trioxide to phosphorus pentoxide of approximately :206. The milling rate for these two mixtures was about 0.85 mil per minute and 1.1 mils per minute on the unmasked portion of the alumina part.l

The part was masked with polytetrafiuoroethylene in a manner previously described. Briefly, a primer coat was applied, cured, the polytetrauoroethylene liquid was sprayed over .the primer coat, dried, and cured. It was noticed that the maskant was not attacked appreciably by the chemical milling solution and that it stood the action of this solution very well for the one-half hour 2i) part is the fact that the chemically milled part does not have as many points of stress. More particularly, in the mechanical-ly milled part the metal is chewed or abraded away so as to leave ridges or raised regions .which are places of stress. In contrast to this in the chemically milled part the action is so uniform over a wide area that the surface of the chemically milled part is substantially smooth and even so that there are no ridges to act as centers of stress.

My process is also adaptable for a continuous method or a batch method. In the continuous process it is possible to contact the structural item to be made with a milling solution and to continuously recirculate the milling solution s0 as to maintain uniformity of solution in contact with all reacting surfaces. Chemical processing equipment, i.e., tanks, pipe iittings, valves, and pumps lined with a suitable resistant material such as polytetrafluoroethylene capable of withstanding the chemical action of these solutions are available. Also, filter screens and heat exchange equipment to withstand the chemical action of these solutions are available. Therefore, it is possible to maintain close solution control necessary for continuous operation.

Examples of other ways and means for chemically -niilling metals are herewith presented. A metal may be treatment. A summary of this data is presented in folremoved by spraying the solution onto it so as to have a lowing Table XIV. line stream oi the reactant solution contact the metal.

TABLE XIV Chemical Milling of a Partially Masked Alumina Part With a Mixture Comprising One Part Fumiz'ng Sulfuric Acid and Three Parts Concentrated Phosphoric Acids and Mercury P205 added Percentby Percentby Maskant Time, Temp. Milling, Milling Weight percent by Weight Weight thickness, min. F, mils rate, mils ratio weight S03 P205 mils per min. SC3/P205 56. 85 13. 6 se. 0 11 30 55o 255 0. 85 0. 206 56. 85 13. s 6G. 0 s 3o 550 34 1. 1 0. 206

The fourth step in this process is the removal of the maskant. As is well known, the maskant Teflon is a hard rubber-like plastic which is substantially inert to acids and alkalis, stands up well under heat and adheres to the part covered. One desirable way to remove the maskant is by sand-blasting. For example, sand-blasting the covered part with a tine sand which has passed through 60 mesh screen removes most of the maskant without harm to the part.

The control for determining the degree of chemical milling may be carried out in a number of different manners. One of these is to introduce a material of a predetermined thickness into the chemical milling solution along with the member to he chemically milled and to periodically weigh or measure this member. When this member has decreased a certain amount in weight or thickness it is possible to decide whether the structural member undergoing chemical milling has been corroded away sui'lciently.

With my process and chemical milling solution it is possible to achieve tolerances in some usages which have not been attainable in commercial practice before. These tolerances are in the range of approximately plus or minus 0.0005 inch, making it possible to achieve a high quality and/or a precision product. Also, in my process it has been observed that there is no intergranular conosion due to the chemical action of the solution. This success in not having intergranular corrosion makes it possible to `achieve higher strength components for less weight of the component as compared with the presently machined milled and welded components.

Also, contributing to the strength of the chemically milled part in comparison with the mechanically milled By this technique deep grooving can be realized by rotating a part. In another manner a pipe may be made lighter-in-weight by running the solution through it so as to decrease the wall thickness. As is realized the interior wall of the pipe is eaten away and the inside diameter thereby enlarged.

From the above description of my discovery it is seen that the same is applicable to the chemical milling of -rnetal and alloys. From my discovery it is possible to save considerable weight in the :manufacture of metal roducts; to produce the parts in batches or continuously; the capital investment for production of the components may be as low as approximately ve to ten percent of the capital investment for equivalent machine milling. Also, it is possible to produce highly complex shapes and configurations with chemical millingwhich are not possible with machine milling, to use non-symF metrical patterns with chemical milling which are diflicult to use with machine milling, to produce integrally stiiened structures whereby one unit serves as the structure instead of an integrally fabricated structure depending upon welds, which are weaker than the metal itself, t0 produce the structure, and to make tapered structural materials which are difficult to make with machine milling. Furthermore, from an engineering design standpoint it is seen that it is possible to etch after forming the structure. In many operations it is economically feasible to obtain closer tolerances, to have a number of various depths of c-ut, and to mill all surfaces of the part simultaneously. In this regard, it is possible to chemically mill with equal ease all types of alumina and compositions of matter containing alumina. From a labor standpoint and skilled artisan viewpoint it is not necessary in chemical milling to use as highly skilled operators as in machine milling.

Having described my discsfvery, what I claim and wish to protect is as follows:

1. An acidic chemical milling composition comprising a phosphoric acid and sulfuric acid and a minor amount of mercury, the phosphorus content expressed as phosphorus pentoxide being in the range of about l%70% by Weight of the composition, the sulfur content expressed as sulfur trioxide being less than about 80% by Weight of the composition, the Weight ratio of sulfur trioxide to phosphorus pentoxicle being from about 1:3 to approximately 8:1, and the amount of mercury being suicient to Substantially decrease the milling time.

2. A process for chemically milling alumina comprising contacting the alumina and the phosphoric acid milling composition at a temperature in excess of 500 F., the phosphorus content of said acid composition expressed as phosphorus ,pentoxide being at least about 62% by Weight of said composition.

3. A process for chemically milling alumina comprising contacting the alumina and a phosphoric acid milling composition at a temperature in excess of 500 F., said milling composition having a minor amount of mercury, the phosphorus content of said acid composition expressed-ias phosphorus pentoxide being at least about 62% by Weight of said composition.

4. A process for chemically milling alumina comprising contacting the alumina and a phosphoric acid milling composition at a temperature in excess of 500 F., the phosphorus content of said acid composition expressed as phosphorus pentoxide being in the range of about 62%-88% of the Weight of said composition.

5. A process for chemically milling refractory metals and metal alloys containing metals selected from the group consisting of nickel, molybdenum, chrominum, iron and `cobalt comprising contacting same and a phosphoric milling composition at a temperature in excess of 500 F., the phosphorus content of said acid composition expressed as phosphorus pentoxide being at least about 62% by Weight of said composition.

6. A process for chemically milling alumina, said process comprising contacting at a temperature in excess of 500 F. the alumina and an acidic composition containing sulfur and phosphorus, the phosphorus content Z2 thereof expressed as phosphorus pentoxide being in the range of about 10%-70% by Weight of said composition, and the sulfur content expressed as sulfur trioxide being less than about by Weight of said composition.

7. A process for chemically milling alumina, said process comprising contacting at a temperature in excess of 500 F. the alumina and an acidic composition containing sulfur and phosphorus, the phosphorus content thereof expressed as phosphorus pentoxide being in the range of about 10%-70% by Weight of the composition, and the sulfur content expressed as sulfur trioxide being less than about 80% by weight of the composition, and said composition also containing a minor amount of mercury.

8. A process for chemically milling alumina, said process comprising contacting at a temperature in excess of 500 F. the alumina and an acidic composition containing sulfur and phosphorus, the phosphorus content thereof expressed as phosphorus pentoxide being in the range of about 10%-70% by Weight of the composition, and the sulfur content expressed as sulfur trioxide being less than about 80% by Weight of the composition, the weight ratio of sulfur trioxide to phosphorus pentoxide being from about 1:3 to 8:1, and said composition also containing a minor amount of mercury.

References Cited in the file of this patent UNITED STATES PATENTS 2,398,212 Durgin Apr. 9, 1946 2,650,156 Shelton-Jones Aug. 25, 1953 2,729,551 Cohn Ian. 3, 1956 2,739,047 Sanz Mar. 20, 1956 2,879,147 Baker Mar. 24, 1959 2,923,608 Margulies Feb. 2, 1960 2,967,136 Cybriwsky et al Jan. 3, 1961 FOREIGN PATENTS 845,590 Germany Aug. 4, 1952 OTHER REFERENCES A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Mellor, published by Lon gmans, Green and Co., London, 1924, vol. 5, pages 270, 271 relied on.

Inorganic Chemistry; 1926, Partington; MacMillan `and Co., Ltd., London; pages 498 and 628. 

5. A PROCESS FOR CHEMICALLY MILLING REFRACTORY METALS AND METAL ALLOYS CONTAINING METALS SELECTED FROM THE GROUP CONSISTING OF NICKEL, MOLYBDENUM, CHROMINUM, IRON AND COBALT COMPRISING CONTACTING SAME AND A PHOSPHORIC MILLING COMPOSITION AT A TEMPERATURE IN EXCESS OF 500* F., THE PHOSPHORUS CONTENT OF SAID ACID COMPOSITION EXPRESSED AS PHOSPHORUS PENTOXIDE BEING AT LEAST ABOUT 62% BY WEIGHT OF SAID COMPOSITION. 